Polyisocyanates containing biuret groups are known and are used starting materials for high-grade, light-stable lacquers. They may be obtained for example, from diisocyanates and water (German Auslegeschrift No. 1 101 394 or U.S. Pat. No. 3,124,605), hydrogen sulphide (German Auslegeschrift No. 1,165,580), formic acid German Auslegeschrift No. 1,174,760), tertiary alcohols (German Auslegeschrift Nos. 1 543 178 and 1 931 055 or U.S. Pat. Nos. 3,358,010 and 4,051,165) or monoamines (German Offenlegungsschrift No. 2,308,015 and U.S. Pat. No. 3,903,127).
In these conventional processes, amino groups are initially formed from some the isocyanate groups and further reacted with excess diisocyanate via the corresponding urea diisocyanates to form biuret polyisocyanates. The conversion of the isocyanate groups into amino groups is always accompanied by the formation of gaseous secondary products, such as carbon dioxide, carbon monoxide, carbon sulphoxide or olefins, whose elimination may give rise to emission problems. In the heterogeneous reaction of diisocyanates with water, an additional problem lies in the formation of insoluble polyureas which are difficult to separate off. However, the particular disadvantage of these known processes lies in the fact that some of the isocyanate groups, in the diisocyanates used as starting material, are initially destroyed as a result of formation of amino groups.
There has also been no shortage of attempts to produce polyisocyanates containing biuret groups by directly reacting diamines with diisocyanates without any elimination of volatile secondary products and without the destruction of isocyanate groups in amine formation.
On account of the high reactivity of aliphatic amino groups to isocyanate groups, considerable practical difficulties were encountered, because the reaction of primary diamines with diisocyanates has a great tendency to form insoluble polyureas and cross-linked products.
As may be seen from German Offenlegungsschrift No. 2,261,065, for example, uneconomically long after-heating at elevated temperature is required for completing the reaction where readily available starting materials, such as hexamethylene diamine and hexamethylene diisocyanate, are used. This greatly impairs the properties of the end products, particularly the natural color thereof.
According to German Offenlegungsschrift No. 2,609,995 and U.S. Pat. No. 4,147,714, these disadvantages may be obviated by introducing the diamine into the diisocyanate in vapor form under carefully controlled conditions. In this process, however, steps have to be taken to ensure that no diisocyanate enters the feed pipe because otherwise blockages attributable to urea formation may rapidly occur.
Processes using certain specific starting materials have also been successful to some extent. Thus, according to German Pat. No. 1,215,365, diaminopolyethers of relatively high molecular weight have been used as the diamine component to prevent the formation of substantially insoluble secondary products. The process according to German Offenlegungsschrift No. 1,963,190 is restricted to the use of diprimary aromatic diamines whose reactivity is reduced by steric or electronic effects. According to British Pat. No. 1,078,390, the formation of insoluble polyureas may be prevented by directly reacting primary diamines with diisocyanates in the presence of a solvent, for example, chloroform. The above process is further restricted to aromatic diamines and also has the disadvantage of the solvent having to be removed on completion of the reaction.
In these conventional processes, formation of the biuret polyisocyanates is accompanied by rearrangement reactions by which the diamine used is converted into the corresponding diisocyanate in dependence upon the NCO/NH.sub.2 -ratio. As a result, mixtures of different diisocyanates accumulate as distillates during separation of the unreacted diisocyanate from the biuret polyisocyanate unless diamines and diisocyanates of the same constitution are used. Another disadvantage is that more or less large quantities of the diisocyanate formed from the diamine by trans-biuretization remain behind as a monomer conponent in the biuret polyisocyanate itself.
According to German Offenlegungsschrift No. 2,010,887 and U.S. Pat. No. 3,862,973, the direct reaction of secondary diamines may be carried out in particular using aromatic diisocyanates. However, products containing aromatically bound isocyanate groups produced by this known process are unsuitable for high-grade, light-stable lacquers.
Accordingly, an object of the present invention is to provide a process by which it is readily possible to product high-grade, modified, aliphatic polyisocyanates which combine the advantages of known biuret polyisocyanates without the process being attended by the above-mentioned disadvantages.